The present invention relates to the field of semiconductor wafer testing and, more particularly, to a non-contact method for testing such wafers.
In the semiconductor industry, the behavior of FET and bipolar devices used to fabricate integrated circuits are very sensitive to induced charges on the silicon wafers. These induced charges can result from static charging of insulator surfaces, poorly formed oxide/silicon interfaces and excessive ionic contamination within the insulator bulk.
The most prevalent source of ionic contamination is sodium. Other less prevalent sources are potassium and lithium. Some sources of sodium can be contaminated quartz ware, incompletely removed photoresist and inadvertent human contact. The common approach to test for sodium contamination is the use of various MOS monitoring techniques. Bias temperature stressing methods are used to electrically quantify the concentration of sodium in insulator layers (usually thermal oxides). The sodium is forced to move down and up in the oxide layer (push-down and pull-up) and then the sodium is either detected as: (1) a change in net charge imaged on the silicon surface (before and after sodium movement) or (2) a change in integrated ionic current (before and after sodium movement) or (3) as a momentary ionic current (during sodium movement).
Although widely accepted, the MOS methods have increasingly unacceptable high cost and excessive time associated with the MOS sample preparation. For monitoring thick oxides, the sample preparation time for aluminum MOS electrodes can be 1-2 days and for thin oxides, the cost and time for fabricating polysilicon electrodes is even worse. Furthermore, the fabrication process for these MOS electrodes can become a source for sodium or other measurement complications.
U.S. Pat. No. 5,498,974, which is incorporated herein by reference, teaches a method and apparatus for measuring mobile charge in an oxide layer on semiconductor wafers using corona charge.
A corona gun is used to deposit a measured quantity of charge on the oxide surface and then a Kelvin probe is used to measure the potential of the oxide surface. The wafer is alternately situated under the corona gun and then under the Kelvin probe until a series of values of potentials are reached.
The mobile charge measurement is based on the difference between the actual charge required to achieve a desired potential and the theoretical amount of charge required for zero mobile charges.